The present invention concerns a liquid metal ion source used for a focused ion beam (FIB) device.
FIG. 1 is a block diagram showing one example of a liquid metal ion source and a control circuit. The ion source comprises a reservoir 1, a filament heater 2, a metal needle 13 and an extraction electrode 3. The liquid metal 14 stored in the reservoir 1 is maintained at a temperature higher than the melting point by the heater 2 and a heater Controller 7 is supplied to the needle 13.
An extraction voltage applied between metal needle 13 and extraction electrode 3 forms an intense electric field near the center of needle 13 to lead out the liquid metal in an ionized state. The ions pass through a small aperture in extraction electrode 3 and are accelerated by a grounded acceleration electrode 4. An acceleration voltage is applied between needle 13 and the acceleration electrode 4 by an acceleration voltage controller 12. The thus led out ions form a beam 6 and are introduced through a small aperture disposed in the acceleration electrode 4 to an FIB optical system.
A monitor aperture 5 is disposed at the FIB optical system, and the amount of ions flowing therethrough is detected by means of a current detector device 10 connected to the monitor aperture 5.
Control for the liquid metal ion source is fed back to the extraction voltage such that the amount of ions flowing into the monitor aperture 5 provided in the vicinity of the ion beam axis is maintained constant. In other words, a feedback controller 9 controls the extraction voltage generated by the extraction controller 8 such that the current detected at monitor aperture 5 and by current detector 10 is maintained constant.
An example of monitoring extraction voltage changes with time is shown in FIGS. 2(a) and 2(b). The region A shows a stable operation state which gives no problem for utilization as a FIB. The region B shows an unstable state and, since the extraction voltage changes in accordance with the variation of the amount of released ions, the tracks of the ion beam are changed and the imaginary image position is changed. Those changes prevent satisfactory functioning of the FIB device.
It is assumed that the unstable operation as in the case of the region B in FIG. 1(a) is attributable to a insufficient supply of liquid metal 14 to the needle 13 as an ion generation point in FIG. 1, which reduces the ion emission level and elevates the extraction voltage.
It is considered that the amount of liquid metal supplied changes for the following reasons. Ions led out from the vicinity of the tip of the needle 13 collide against the extraction electrode 3 or other electrodes, whereupon metal atoms (or molecules) emitted by sputtering caused by the collision are vapor deposited on the needle tip or the surface of the liquid metal, and/or residual gas molecules or other obstacles are absorbed and deposit on the needle tip or the surface of the liquid metal at the tip to form contaminations. The above phenomena increase the flow resistance encountered by liquid metal flowing along the surface layer of needle 13 near the tip.
Further, mixing of such a liquid metal with a different kind of metal causes the melting point or the viscosity to change, resulting in failure to obtain a stable ion beam, and, in an extreme case, termination of generation of ions per se.
It might be noted that as concerns the operating metal ion source used in FIB apparatus, a lower emission current (0.1 to 10 .mu.A) and lower temperature (about from melting point to +200.degree. C.) are advantageous for obtaining a beam diameter of sub-micron order since the spread of energy is small, as described in the literature "J. Appl. Phys. 51, 3453-3455 (1980)". The emission current is the sum of the ion beam current and is detected by an emission current detector 11 connected with the acceleration controller 12. In particular, in the case of an FIB mask repair apparatus or an FIB device fabrication apparatus, the acceleration voltage applied between the needle 13 and the acceleration electrode 4 is within a range between several kV and several tens kV; this is an indispensable operation condition.
However, in the operation under such conditions of low emission and low temperature, the liquid metal 14 flows slowly along the needle tip and many impurities are likely to be absorbed or vapor deposited. That is, the above mentioned conditions would likely cause unstable operation.